An embodiment is a thin film transistor which includes a gate electrode layer, a gate insulating layer provided so as to cover the gate electrode layer; a first semiconductor layer entirely overlapped with the gate electrode layer; a second semiconductor layer provided over and in contact with the first semiconductor layer and having a lower carrier mobility than the first semiconductor layer; an impurity semiconductor layer provided in contact with the second semiconductor layer; a sidewall insulating layer provided so as to cover at least a sidewall of the first semiconductor layer; and a source and drain electrode layers provided in contact with at least the impurity semiconductor layer. The second semiconductor layer may consist of parts which are apart from each other over the first semiconductor layer.